Pulmonary congestion is a clinical condition that is caused by a number of different diseases such as heart failure or kidney disease. It consists of an accumulation of fluid in the interstitial and alveolar space of the lung following increased blood pressure in the pulmonary capillary vessels that leads to leakage of water from the blood to the lung space. It causes fluid retention and fluid redistribution in the body and leads to symptoms like dyspnea, fatigue, and activity intolerance. Pulmonary congestion resulting from elevated left atrial and left ventricular filling pressures is a main reason for heart failure hospitalization. This condition has a progressive nature and clinical signs and symptoms of pulmonary oedema occur late, typically when the lung fluid has increased at least six-fold from the initial stage of interstitial oedema. This means that pulmonary oedema is often not detected early, and necessary treatment for the patient is delayed.
Bio-impedance measurements, obtained by a bio-impedance monitor (BIM) using, for example, external electrodes or an implanted device to measure the resistance of biological tissue to a small alternating current flowing across a region of interest, e.g. the thorax, can be used to detect pulmonary congestion. The principle underlying this technique is the fact that the electrical impedance (resistance and reactance) of biological tissue is directly linked to the hydration and water content of the tissue, namely intra-cellular and extra-cellular water. When thoracic fluid accumulates (e.g. during pulmonary congestion), there will be a better conductance of the current resulting in a decreased impedance. By measuring the impedance at different frequencies the resistance of the extracellular water (Re) can be estimated according to the Cole-Cole model. Therefore, measurements of the electrical properties of the tissue can indicate the amount of fluid present in that part of the body.
If external electrodes are used, impedance measurements are influenced by several factors including sensor placement, skin moisture, body dimensions and body posture. It has been found that body dimensions and fat mass are particularly relevant to thoracic impedance measurements, making such measurements subject-specific. Implantable devices are not affected by variations in electrode placement or skin moisture; however, it is known that the measurements made by such devices have variability due to less controlled measurement conditions (the patient may be unaware that a measurement is being taken and so measurements may be obtained in a variety of situations which would cause differing fluid distribution in the body, such as lying down, sitting, walking, or exercising). These factors, combined with the normal variability of bio-impedance measurements, make it challenging to determine when an impedance measurement for a specific patient is abnormal and thus indicative of excess fluid accumulation.
“Intrathoracic Impedance Monitoring in Patients with Heart Failure” by Yu et. al., Circulation 112 (2005), 841-848 describes an algorithm that can be used to detect abnormal measurements. The algorithm estimates the baseline impedance (BL) and the short term impedance (STA) on the basis of daily bio-impedance measurements. The baseline acts as a reference that is not sensitive to large deviations of the new measurements. On the other hand, the STA is a filtered estimate of the recent measurements and is more sensitive to large deviations of the measurement. The algorithm raises a flag if the cumulative sum of the difference BL-STA exceeds a certain threshold for consecutive measurements, where BL-STA>X. The disadvantage of the described algorithm is that it does not consider the subject-specific variability of the impedance measurements, e.g. X is constant for each patient. As noted above, it is known that the measurement variability is patient specific, which implies that this method is not optimal and prone to false alarms.
A reliable means of fluid content monitoring would be a valuable tool to improve outcomes in heart failure hospitalizations and reduce healthcare costs. There is therefore a need for an improved method and apparatus that can provide a reliable determination of whether a bio-impedance measurement obtained for a given subject is abnormal for that subject, and thus indicative of ill-health (for example indicative of excessive fluid accumulation in a part of the body where the resistance is relatively low, or, on the other hand, indicative of dehydration of the subject where the resistance is relatively high). Such a method and apparatus could be used in a home or hospital-based monitoring system to detect the presence and progression of pulmonary congestion, as well as for monitoring improvements in the patient's condition as a result of receiving treatment. Such a method and apparatus could also be used in monitoring other physiological characteristics of a subject, such as weight, heart rate, blood pressure, temperature, etc., where low or alternatively high values of the physiological characteristic indicates that the subject has (or the degree to which the subject has) a physiological condition.